Although they can't... they have a reason for dropping
you.
Looking at the subjects they are forcing you to drop, are you doing
well in them? And did they offer you an alternative? For A Maths,
there's no alternative. For Science, did they offer you to do
Combined Science?
The offering of alternatives is important, because there's a reason
for doing so.
These subjects require more work... don't do well... the school
might jolly well force you to drop, whether or not you paid and
registered. Bringing up to MOE or SEAB might not help, because your
results can't prove that you are doing well.
Note that this has nothing to do with pulling the whole cohort's
results down, but rather an individual's interest.
Unless you are doing well... then bring it up to MOE or SEAB.